"The move soak, introduced in Generation 5, is a reference to that battle with Onix. The move soaks the opponent with water, changing it into a water type. This, by extension, makes the pokemon weak against electric attacks." - Chthulhu

Dude, as true as what I said is, xturboxx is right. In game, electric couldn't damage ground at all. For this reason, Yellow version made it so male Nidorans were given access to double kick early, and Mankeys could be found much earlier than in Red/Blue/Green

The move soak, introduced in Generation 5, is a reference to that battle with Onix. The move soaks the opponent with water, changing it into a water type. This, by extension, makes the pokemon weak against electric attacks.

Arcanine is probably on the door, because originally, Arcanine was supposed to be grouped with Articuno and Zapdos. He was replaced with Moltres, because gamefreak wanted the three legendaries to all be birds. Him being there is most likely a reference to how he was replaced

Entei wasn't created yet, it really was arcanine because arcanine was originally going to be the fire legendary instead of Moltres, except, tey thought it would be odd to have two birds and a dog. So they made growlithe and moltress but arcanine still kept it's high status in the pokemon world and ancient times in the anime and so it's pictured here.